High-performance fibers are being increasingly used as the reinforcement of plastic, metal, ceramic, and carbon matrix composites. When the composite has a ceramic matrix, the main role of the fibers is to toughen the composite to prevent brittle failure. The degree of toughness attained is greatly affected by the bond strength between the fibers and matrix. If the bond strength is too high, cracks propagate through the fibers; if too low, the load is not transferred to them.
The most demanding of these applications are those involving high operating temperatures. In such environments, the matrix may chemically react with, or dissolve the fiber. Although chemical reaction may in some cases be beneficial, it usually leads to drastic reductions in strength and toughness. In many cases, these high-temperature problems can be solved by applying barrier coatings on the fibers by chemical vapor deposition (CVD). As the name implies, CVD involves the deposition of coatings onto substrates by chemical reaction from the vapor phase. The technique is widely known and a number of review articles exist on the subject. See, for example, Blocher, J. M., Jr., "Deposition Technologies for Films and Coatings", Noyes Publications, page 335-364 (1982). Blocher discusses the roles of thermodynamics in predicting the possibility of deposition with various reactants under given temperatures and reactant partial pressures, of kinetics on the rate of deposition, and of transport processes such as diffusion and heat transfer in CVD. This reference also describes the effects of these variables and their interactions on coating properties.
The application of a coating by CVD to a monofilament is a fairly simple procedure. See, for example, EPO Pat. Publ. No. 0,222,960 (Schachner) where a monofilament is drawn and coated by CVD in-line. When the monofilament is electrically conductive, it can be heated resistively during CVD, thereby activating the chemical deposition reaction primarily on the monofilament and not in the gas phase or on the unheated wall of the tube that confines the gaseous mixture. There is a considerable amount of art describing such "cold wall" deposition systems by which CVD coatings are applied to monofilaments by continuous processes. See, for example, U.S. Pat. Nos. 3,549,424 and 4,068,037.
When coating by CVD a fiber that has multiplicity of filaments, e.g., a tow, it is necessary no diffuse the reactant(s) between the filaments. A translation of French Patent No. 2,607,840 states that:
"This vapor phase deposition, designated by the American acronym CVD, does not allow one to obtain good protection for all of the individual filaments that constitute the tow of carbon. Actually it is very difficult, if not impossible, to avoid preferential deposition, particularly in the peripheral zones of the tow, and equally, to avoid the cementing together of the filaments. These drawbacks make the process unsuitable for industrial utilization. In other words, the CVD technique, even though very attractive in theory, does not allow the control of coating thickness and homogeneity of the carbide deposit specially when, as is the case in a tow, the gaseous reaction medium diffuses poorly into the center. It follows that the individual filaments of carbon are not coated homogeneously, regularly, and with controlled thickness." PA1 a straight, elongated furnace tube extending through the furnace, which furnace tube is formed with PA1 a. while excluding the atmosphere, continuously carrying the fibrous material lengthwise at atmospheric pressure through a heated gaseous mixture comprising one or more reagents that deposit CVD coatings, PA1 b. freely exhausting the residue of the gaseous mixture along the path of the fibrous material in the direction of its movement, PA1 c. maintaining the fibrous material within the gaseous mixture for a time to deposit a CVD coating onto the moving fibrous material, and PA1 d. removing the coated fibrous material from the gaseous mixture.
To alleviate the difficulties mentioned in the French patent, it is common to coat multiple-filament fibers by CVD at low pressures (LPCVD). This increases the mean free path of the reactants, thereby decreasing homogeneous nucleation and the growth of soot particles in the gas phase. It also facilitates the diffusion of the gases between the filaments, thus reducing the variability in coating thickness. However, such a reduced pressure coating process requires building and maintaining of sophisticated, expensive equipment. See U.S. Pat. No. 4,343,836 (Newkirk et al.) In many cases, such equipment is limited to batch processes. See, for example, U.S. Pat. No. 3,212,926 (Morelock) and U.S. Pat. No. 4,214,037 (Galasso et al.), the latter of which suggests that similar coating results can be obtained in a continuous process.
Difficulties are encountered when LPCVD processes for coating a multi-filament fiber are made continuous. Deposits on the inner wall of the coating chamber gradually diminish its volume and eventually require it to be replaced. Fuzz (tangles of broken off filaments) and soot (homogeneously nucleated and grown particles) that form in the system during coating interfere with fiber movement. To prevent fiber breakage, the fuzz and soot must be periodically removed. When cleaning is required or when the fiber breaks during the coating process, the vacuum has to be broken, and after repair the system has to be pumped down. If the coating is performed at high temperatures, partial cool-down and subsequent reheating of the coating system is also required.
These difficulties demonstrate a need for a system by which a multi-filament fiber can be continuously coated by CVD at atmospheric pressure (APCVD), and such systems have been reported. See, for example, Honjo et al., Composite Interfaces, Proc. Int. Conf. 1st, pp. 101-107 [1986]; Aggour et al., Carbon, Vol. 12, pp. 358-362 [1974]; and Amateau, J. Compos. Mater., Vol. 10, pp. 279-296 [1976]). However, the apparatus illustrated in each of those publications would need to be disassembled to be cleaned and so would offer little advantage over a LPCVD system such as that shown in the Newkirk patent.
U.S. Pat. No. 4,373,006 (Galasso et al.) says that "even when upwards of 10,000 fibers are bundled together to form a strand of yarn the chemical vapor deposition of silicon carbide produces an essentially uniform coating of silicon carbide over the surface each fiber even on those fibers in the center of the yarn and even on those areas of fibers which are in close proximity to one another" (col. 2, lines 57-64). It also says that carbon fibers were coated with silicon carbide "by holding the fibers in a chamber . . . maintained at a temperature of between 1100.degree. and 1200.degree. C. by passing them through an R.F. heated graphite susceptor" (col. 3, lines 41-49). However, there is no disclosure of the nature of the fiber-handling apparatus and no drawing.
For a detailed discussion of the advantage of coating ceramic fibers with BN for use in composites, see U.S. Pat. No. 4,642,271 (Rice). However, coating conditions are not given. It is not even stated whether APCVD or LPCVD is used, and no apparatus is illustrated.
U.S. Pat. No. 4,731,298 (Shindo et al.) concerns coating carbon fibers first with a layer of carbon and then with a metal carbide. "The carbon fibers may be in the form of yarns, tows or webs of continuous filaments. The carbon fibers may be used in the form of yarn and webs of short fibers or the like" (col. 2, lines 65-68). However, no method is disclosed; neither is any apparatus disclosed.